Drier for bulk material

ABSTRACT

A machine for converting moisture-laden bulk material into dried granular condition. Material emerging from the drying area is mixed with raw material to maintain a consistency and density related to the capacity of the drier to extract moisture. The recirculation also tends to maintain a predetermined particle or granule size, as the &#39;&#39;&#39;&#39;fines&#39;&#39;&#39;&#39; are re-agglomerated in the mixing with raw material. The drying action is provided by supplying heat to a chamber containing baffles over which pulverized material is uniformly spread by a rotary distributor whereupon the material flows across such baffles by gravity assisted by vibration. The material is hurled upwardly to the uppermost baffles through a central duct leading from the pulverizer, and particles in excess of a given size are returned in a central conduit preferably surrounding the duct.

[ Oct. 16, 1973 4] DRIER FOR BULEMMATERIALWH [75] Inventor: Harvey Wenger, Holland, 7

..Ml hr.

[73] A s s ig iiee: Cdnti' ol Corporation, Grand Haven, Mich. [22] Filed: Jan. 7, 1972 [21] Appl. No.: 216,038

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 846,455, July 31,

1969, abandoned.

[52] U.S. Cl 241/23, 241/24, 241/34,

[51] Int. Cl. B02c 21/00 [58] Field of Search 241/23, 34, 60, 61, 241/65, 80, 97; 34/102, 164, 166, 167, 171,

[56] References Cited V UNITED STATES PATENTS 2,090,187 8/1937 Credo 241/61 X 2,207,360 7/1940 Spellacy 241/61 2,835,452 5/1958 Cline et al. 241/23 MOISTURE SENSING CONTROL Primary Examiner-Granville Y. Custer, Jr. Attorney-William R. Jacox et a1.

[5 7] ABSTRACT A machine for converting moisture-laden bulk material into dried granular condition. Material emerging from the drying area is mixed with raw material to maintain a consistency and density related to the capacity of the drier to extract moisture. The recirculation also tends to maintain a predetermined particle or granule size, as the fines are re-agglomerated in the mixing with raw material. The drying action is provided by supplying heat to a chamber containing baffles over which pulverized material is uniformly spread by a rotary distributor whereupon the material flows across such baffles by gravity assisted by vibration. The material is hurled upwardly to the uppermost baffles through a central duct leading from the pulverizer, and particles in excess of a given size are returned in a central conduit preferably surrounding the duct.

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PATENTED'ncr 16 um SHEET 9 [IF 9 DRIER FOR BULK MATERIAL BACKGROUND OF THE INVENTION This application is a continuation-in-part of my application Ser. No. 846,455 filed July 31, 1969, now abandoned and assigned to the same 'assignee as this application. The First Embodiment herein essentially consists of the drier disclosed in my application Ser. No. 846,455, now abandoned in favor of the present application. The Second Embodiment represents a drier similar to the First Embodiment but incorporating numerous improvements.

Bulk material driers have been developed in conjunction with systems for conditioning organic wastes for use as fertilizer. Sewage plants and large-scale cattle or poultry operations have established that a material once considered as a disposal problem can become a source of revenue. The development resulting from interest in this field has been found applicable to a much wider area including chemicals and food products.-

Conventional driers make use of heat supplied to processing chambers in which material is separated or spread out to provide exposure. Standard forms of conveyors usually deliver the raw material to a pulverizer or some other form of preliminary conditioner, following which the material is delivered to the drying chamber. Machines of this type display an objectionable tendency to function properly only within a very limited range of variation in the original moisture content of the raw material. A mud-like consistency can easily produce such a sticky mass within the machine that it does not break down well into particle size such as to provide adequate mass exposure to the heat, and the adhesive quality of many forms of this material tends to cause it to stick to practically any exposed surface. As a result of these tendencies, some attempts have been made to precondition the raw material to a given range of moisture content by pressing, centrifuging, or some other such operation that will place the raw material in a condition in which it can be handled by the conventional drier. One incentive to the development of suitable driers centers in the commerical value of the resulting fertilizer. The alternative to drying and marketing these wastes is to treat them as a disposal problem, distributing them through lagoons or drainage areas of sufficient capacity to break down the solids to the point that they can be absorbed by the plant growth. Chemical processing stations have also been utilized, in which the primary function is to convert the waste products into something that can be discharged into the drainage systems witout contaminating them, and without producing objectionable odors.

SUMMARY OF THE INVENTION The present invention eliminates the dependence of the machine upon the existence of a limited range of moisture content in the raw material by recirculating the output of the drying chamber, mixing this output with the raw material at a rate to produce a consistency such that the drying equipment can handle it effectively and produce a desired particle size. The rate of delivery of raw material to the machine can be controlled manually, or it can be done automatically in several ways, such as by making the delivery mechanism directly or indirectly responsive to variations in the moisture content of the drier output.

In the operation of the drier of the present invention raw material is deposited in a mixing bin from which the contents are delivered to a pulverizer associated with the drying chamber. The pulverizer discharges the material it processes at a sufficient velocity to carry it upwardly through a duct, whereupon the material is discharged in a fairly even layer over the uppermost of a series of baffles associated with a vibrating chamber structure. A screen controls the maximum size of particles admitted to the drying chamber, the remainder passing down through a conduit surrounding the duct to return oversized particles to the pulverizer for further pulverization. If it becomes desirable to maintain a minimum, as well as a maximum, particle size, fines can be screened out of the drier at the output stage and returned to the mixing bin, where exposure to the moist raw material will cause the fines to reagglomerate.

The drier of the present invention pulverizes the moist raw material to a relatively fine condition. After it is spread out in a circular pattern on the heated upper tray, the vibrations of that tray cause the finely pulverized moist raw material to tumble down the tray during which time it agglomerates into small somewhat spherical granules. As these granules drop through the screen, they are subjected briefly to a relatively high temperature (eg. on the order of 700F. This brief exposure to very high temperatures is sufficient to dry and sterilize the particles to desired levels without driving off all the nutrients. Nitrogen, potassium and phosphorous, for example, are three essential elements for plant nutrition which can be driven off in sufficient amounts upon exposure of animal waste to high temperatures for prolonged periods of time so as to render the material relatively worthless as a fertilizer. The drier of the present invention employs rapid drying to minimize nutrient retention while reducing moisture content to acceptable levels (normally considered to be about l0-15 percent moisture content).

A moisture content of about 35 percent is considered the optimum condition for raw material entering the pulverizer. Materials having moisture content below about 20-25 percent lack sufficient adhesive properties to agglomerate from a finely pulverized condition into granules. Materials having moisture content above about 40 percent are too adhesive and tend to clog the system. Accordingly, the present invention contemplates that raw materials having less than about 20-25 percent moisture content must be combined with liquids or other higher moisture content material in the mixing bin prior to entering the pulverizer. Raw materials having moisture content in excess of about 40 percent must be mixed with drier materials and normally this will be done by adding previously dried materials from the drier output to raw material in the mixing bin before conveying it to the pulverizer.

By exposing the wet raw material to high temperatures for relatively short periods of time (eg. on the order of 1-2 minutes), relatively large quantities of noxious by-product gases are emitted from the drying furnaces. For this reason, the present invention contemplates the use of an afterburner so that higher temperatures can be achieved elsewhere than in the drying chamber to incinerate unwanted end products. The present invention further contemplates the use of a heat exchanger to extract heat from the flue downstream of the afterburner and means for returning heat from the heat exchanger to the drying chamber in order to minimize heat loss and retain fuel and other operating cost at economic levels.

Since it is a principal obeect of the invention to turn waste materials from an often valueless and sometimes costly disposal problem into a valuable and, hopefully, marketable product, special attention has been directed to acheiving a uniform, dry, and relatively clean end product at a reasonable cost. In this regard, particular care has been taken to obtain efficient fuelutilization, to avoid unnecessary maintenance, to minimize offensive stack emissions, and to hold the labor cost of operation to a minimum. The manner in which this and other objects have been accomplished has been discussed to some extent above and will become further apparent upon examination of the remaining portions of the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exterior elevation of a drying machine embodying the present invention.

FIG. 2 is a section on an enlarged scale through the central area of the installation shown in FIG. 1.

FIG. 3 is a plan view of the machine shown in FIG. 1, on an enlarged scale.

FIG 4 is a section on a horizontal plane below the dryng chamber of the machine.

FIG. 5 is a horizontal section through the upper portion drying chamber.

FIG. 6 is a section on the plane 6-6 of FIG. 4.

FIG. 7 is a section on the plane 7-7 of FIG. 4.

FIG. 8 is a section on the plane 8--8 of FIG. 4.

FIG. 9 is a section on the plane 99 of FIG. 4.

FIG. 10 is a section on the plane 10-10 of FIG. 5.

FIG. 11 is a fragmentary sectional elevation showing the functioning of the return conduit interconnecting the drier output conveyor with the mixing bin, under conditions in which material is supplied through the conduit.

FIG. 12 is a section similar to FIG. 11, showing the conditions in which the return conduit to the supply bin is not operative.

FIG. 13 is an exterior elevation of the housing of the machine.

FIG. 14 illustrates a return duct usable in conjunction with a burning device installed for removing combustable gases.

FIG. 15 is a longitudinal section elevation taken through a central area of a second embodiment of a drier installation according to the present invention.

FIG. 16 is a transverse elevation of the raw material supply, mixing and heat exchange zones of the drier installation shown in FIG. 15, portions being broken away and shown in section.

FIG. 17 is an enlarged front elevation of the hammermill blade construction employed in the drier installation of FIG. 15.

FIG. 18 is a side elevation of the hammermill blade shown in FIG. 17.

FIG. 19 is a detailed longitudinal sectional elevation on an enlarged scale of the hammermill and sub-frame mounting shown in FIG. 17.

DESCRIPTION OF FIRST EMBODIMENT The complete drier installation shown in FIGS. 1 and 2' has a raw material conveyor 20 positioned to dump its discharge into the mixing bin 21 associated with the supply conveyor 22. The raw material conveyor 20 is of the endless belt type, and will transfer the animal waste from some convenient station into the mixing bin at a predetermined or controlled rate. The supply conveyor 22 is, of the auger type, and transfers material from the bin in a generally horizontal direction to the pulverizer 23 centrally located within the housing 24. Power to drive the auger supply conveyor 22 originates in the motor 25, from which torque is delivered to the auger through the transfer system 26. The motor 25 also drives the mixing shaft 27 provided with the radially extending paddles 28 for agitating and intermixing the contents of the bin 21.

The pulverizer 23 is a conventional device with a horizontal axis rotor inducing a considerable tangential velocity in the particles created by the pulverizing action, and these are discharged by the pulverizer upwardly through the duct 29 which rotates slowly on a vertical axis so that the laterally turned discharge portion 30 at the upper end of the duct spreads the material carried by the duct around the outer pheriphery of the conical receiver 31. The upper extremity of the duct 29 is positioned through the engagement of the journal 32 with a suitable bearing 33 mounted on the shield 34 attached to the conical sheet metal ring 35 forming the open top of the heating chamber. This chamber is defined by the annular shroud 36 attached to the peripheral edges of the receiver 31 and the conical collector 37. Conical baffle plates 38-41 are mounted within the drying chamber, and are secured to the tubular central column 42. Pulverized material thrown to the outer periphery of the receiver 31 will move radially inward to the conical screen section 43 of the receiver, and material passing through the screen will pass in succession over the baffles 38-41 to the collector 37. Particles too large to pass through the screen 43 will move inward and return to the pulverizer by dropping downward through the inside of the column 42.

The movement of the particles down from the receiver to the collector is assisted by a vibratory action. The central column 42 is supported on a sub-frame generally indicated at 44 suspended from a group of bolts 45 carried by a main supporting frame 46. A resilient support for the sub-frame 44 is obtained by interposing springs 47 between the nuts 48 and the subframe. A gentle vertical oscillation at preferably about 325 cycles per minute is generated by eccentric interconnections at 49 and 50 between the ends of the shaft 51 and the legs 52 and 53 of the sub-frame. The shaft 51 is rotatably mounted in brackets 54 and 55 that are fixed with respect to the main frame 46. The shaft 51 is driven by the belt 56 from a suitable pulley mounted on the shaft of the motor 57. This motor shaft, through suitable couplings, delivers power to the pulverizer 23. The pulverizer, the motor, and the pulverizer hopper 58 are all fixed with respect to the main support frame 46. The rotating vertical duct 29 is carried by suitable bearing structure 59 spanning the top of the hopper 58, and the drive for rotating the duct 29 is delivered through the ring gear 60 and the spur gear 61. The latter is mounted on the shaft 62 supported in bearings 63 secured to bracket structure attached to the main frame 46. The shaft 62 is driven by the chain 64 from a sprocket on the shaft of the motor 57. The result of this form of support of the rotating duct 29 is to produce relative oscillations between the duct and the structure of the drying chamber. This relative motion is accommodated by an axial fredom freedom movement of the journal 32 with respect to the bearing 33.

Particles of material working their way down over the baffles 38-41 to the receiver 37 are exposed to heat generated by the gas burner unit 65 mounted within the drying chamber. Partially or fully dried particles reaching the,collector 37 will move downward toward the central opening 66 around the periphery of the column 42. A group of triangular gusset plates as shown at 67 and 68 in FIG. 10, maintain the spaced relationship of the components, and provide support for the column 42 with respect to the sub-frame. Material falling through the opening at 66 will be received in the hopper 68, which communicates with the discharge conveyor 69. This conveyor has a vertical return conduit 70 arranged to deliver material from the hopper 68 to the mixing bin. The consistency of the material in the bin 21 can thus be maintained as a balance between raw material delivered by the conveyor and the dried material brought out by the conveyor 69. The feed conveyor 22 will therefore deliver the desired consistency of material to the pulverizer 23 which can best be accommodated by the pulverizer and the drying chamber to avoid excessive stickiness and obtain the proper granular formation of the particles.

It is preferable to incorporate in the discharge conveyor 69 a moisture-sensing device 69a which will activate the raw material conveyor 20 only when the moisture in the material emerging from the conveyor 69 is below a predetermined amount. A rise in the moisture content of the material in the discharge conveyor 69 shows that the moisture has not been removed, in the drying chamber, and the amount of new moisture added by the raw material must be restricted. As the machine continues to function under these conditions, material drawn out of the hopper 68 by the conveyor 69 will be returned through the conduit 70 as soon as the level in the bin 21 falls below the bottom of the conduit 70, whice is the condition shown in FIG. 11. In this manner, a quantity of at least partially dried material is maintained in recirculation to establish the desired consistency of material moving through the machine. As the level in the mixing bin rises, the bottom of the return conduit 70 is covered. This results in a stoppage of the discharge flow through the conveyor 69 to the unloading conduit 71, which deposits properly dried material in the drum 72. It is desirable to keep a certain amount of dried material within the machine so that starting of the machine does not present the problem of dealing with 100 percent raw material, which will often be of excessive moisture content to be handled properly in the machine. An alternative to this arrangement is to utilize a charge of some convenient and fairly dry mass for purposes of being intermixed in the bin 21 with the raw material involved.

Gases and vapors present in the dying chamber as a result of the heat provided by the burners 65 are free to move upward through the screen 43. Heat loss is minimized by the presence of a layer of insulation 73 surrounding the combustion chamber, and secured to the housing 24. An annular space between the insulation and the shroud 36 provides for the passage of a small quantity of air which proceeds upwardly around the drying chamber and emerges in the space 74, from which it proceeds upward through the exhaust duct 75. It is preferable that'temperatures be maintained which are in excess of the condensation point of the various vapors involved, in order to minimize corrosion. ternperatures within the drying chamber are preferably maintained just below the flash temperature of the particular material being processed. This temperature prevails at the upper extremity of the drying chamber, and reduces to a point somewhat above room temperature at the collector 37. The particle size is preferably interrelated with the temperatures so that the lowest temperature in the central area of each particle is sufficiently high to kill the bacteria that may be involved in the particular material. The high temperature is determined to prevent the formation of ash, and each particle will exhibit a temperature gradient from inside to outside that will be a function of the temperaturetransfer characteristics of that material. Normally, it is preferable to regulate the particle size to that which will pass through an eight mesh screen, and not pass through a thirty-two mesh screen. The reagglomeration characteristics of the material discussed previously can be utilized to assure the production of a granulation within these limits, on the assumption that the fines are screened out of the conveyor 69 and returned to the mixing bin. Frequently, the screen-out is unnecessary, as the fines will tend to agglomerate completely during the passage down through the drying chamber as the particles roll across the surfaces of the baffles 38-41.

Removal of noxious and inflamable gases carried by the exhaust duct can be accomplished by the use of a burner unit positioned as shown at 76. Depending upon the nature of the gases carried by the exhaust duct, this burner can be primarily a source of additional combustion air, or may provide an auxiliary supply of fuel gas delivered in sufficient quantity to raise the temperature above the combustion point of the gases involved. The resulting gaseous mixture is collected by the plenum 77, and may be recirculated to the housing 24 to conserve heat by the arrangement shown in FIG. 14. The duct 78 returns the hot gases to the space within the housing 24, which tends to elevate the temperature surrounding the drying chamber, rather than exposing it to a surrounding gaseous environment at room temperature.

DESCRIPTION OF SECOND EMBODIMENT The second embodiment of the drier installation of the present invention shown in FIGS. 15-19 has a raw material conveyor 80 positioned to dump its discharge into a mixing bin 81 associated with a supply conveyor 82. The raw material conveyor 80 illustrated is of the auger type, driven by a motor 83 and transfers raw material from a source raw material pit 84. It will be apparent that various other types of raw material conveyors can be employed and that the raw material can be conveyed thereby from other places, such as by an endless belt conveyor system-conveying animal waste directly from the animal stalls or shelter. The raw material conveyor 80 transfers waste material from the pit 84 at a predetermined or controlled rate which depends on and is automatically controlled either directly orindirectly by the condition of the material itself at some point prior to its ultimate discharge from the drier system.

The supply conveyor 82 is of the auger type, and transfers material from the bin 81 in a generally horizontal direction to the pulverizer 85 centrally located within the main drier housing 86. The supply conveyor 82 is driven by the motor 87 through the transfer system 88.

Material discharged from the raw material supply conveyor 80 into the mixing bin 81 is churned by a horizontal axis rotary mixing shaft 89 which has a plurality of radially extending paddles 90, at least some of which are set at an angle so as to drive raw material in the bin 81 axially towards the mixing bin discharge opening 91 through which the raw material passes downward into the supply conveyor 82.

The pulverizer shown in detail in FIGS. 17-19 is of the hammermill variety, comprising a shaft 92 journalled for rotation about a horizontal axis. A cylindrical pulley drum 100 is fixedly mounted on the shaft 92 for coaxial rotation therewith. Four circumferentially spaced hammer members are mounted 90 apart on the pulley drum 100, each hammer member consisting of a pair of hammer arms 93 mounted at the ends of the pulley drum 100 and an axially extending hammer blade 96 connected between the outer ends of each pair of hammer arms 93. The hammer blades 96 are provided with tungsten alloy cutting edges 97 which can be removed and replaced when they become worn.

The pulverizer is rotated at a high velocity and the tip speed of the cutting edges preferably approximates about 120 miles per hour, inducing a high tangential velocity in the particles created by the pulverizing action of the blades 96. These particles are discharged from the pulverizer 85 upwardly through the stationary duct 98 and into the rotary particle distributor box 99 at the upper end of the duct 98.

In order to minimize wear on the hammermill and its drive mechanism, the hammer arms 93 are pivotally mounted on the pulley drum 100 and will yield circumferentially when they strike hard or otherwise resisting objects in the material to be pulverized.

In order to most efficiently pulverize and throw the material up the duct 98, it is important to maintain a relatively close tolerance betweenthe shoe 101 and the hammer blades 96. Accordingly, the shoe 101 is provided with a screw adjustment mechanism 102 to compensate for shoe and/or blade wear.

It is important to recognize when the hammermill becomes worn so that pulverized material leaves the hammermill at lower speeds, the material is likely to adhere to the sides of the vertical duct or at least to be sufficiently slowed so that the incidence of fall-back is materially increased, both of which have an adverse effect upon the systems efficiency.

The distributor box 99 is centrally suspended from the top of the main drier housing 86 by suspension drive rod 103 and is driven in rotation about a vertical axis by the motor 104. The distributor box is open at the bottom and is positioned directly above vertical duct 98 to receive discharge therefrom. Deflector plate 105 is mounted in the box 99 at an angle approximately 45 from horizontal. The upper and lower side edges of the plate 105 are spaced from opposed side walls of the box 99. Both the upper and lower edges of plate 105 extend beyond the vertical plane of the sides of the duct 98. Discharge entering the box 99 from duct 98 impinges against the lower surface of plate 105 and is deflected toward the space 107 between the upper edge of the plate 105 and the adjacent box side wall. Most material passing through space 107 falls on the upper inclined surface of the plate 105 and tumbles down plate 105 toward the space 108 between the lower edge of the plate 105 and the adjacent box side wall. Material passing through space 108 lands on the uppermost drying tray.

The distributor box 99 rotates at speeds between 30 and rpm. so that discharge is urged off the upper surface of the plate 105 and through space'108 by centrifugal force and has a reduced tendency to clog the system by sticking on the upper plate surface or in the space 108. It will be noted that the lower surface of the plate 105 tends to remain clean as a result of the continual impingement of high speed discharge material against it, as does the space 107. The top of the distributor box 99 is suitably spaced from the top edge of the deflector plate 105 so that few if any particles of discharge material reach it with sufficient force to adhere to it. To the extent that material does adhere to the top of distributor box 99, the top is spaced a sufficient distance above the upper edge of deflector plate 105 to avoid interference with normal operation. When problems of this type occur with particularly adhesive materials, a taller deflector box 99 is employed.

A small deflector 109 is positioned inside the distributor box 99 at its lower edge beneath the space 108. Material which fails to pass through space 108 falls back against deflector 109. The free edge of deflector 109 ends at approximately the vertical plane of the vertical duct 98 so that fall-back material striking deflector 109 re-enters vertical duct 98 and is returned to the stream of material in the duct coming from the pulverizer and will tend to be thrown back against and over the deflector plate through space 107.

Material thrown up vertical duct 98 from the pulverizer does not normally tend to flow up the center of the duct 98. Instead, the pulverized material has a tendency to flow up one side or another of the duct 98. Thus rotation of the distributor box 99 has the added advantage of distributing discharge material on upper tray 106 evenly on all sides of the duct 98. Even distribution of material on the drying trays is important for efficient operation of the system as a whole.

The drier portion of the system consists principally of the main outer housing 86; a main support frame 110 or base; a sub-frame lll suspended from the support frame 110 for oscillatory movement; drying trays or baffles 106, 112 and 113; a foraminous particle sorting member consisting of a wire mesh screen 115; a cylindrical particle return conduit column 116 through which particles too large to pass through the screen 115 are returned to the pulverizer 85; radiant gas burners 117; and annular shroud 118 which prevents heat from the burners from by-passing the sorting screen 1 15; and a conical downwardly converging collector 114 having a central opening disposed to deposit material in a hopper 126.

The main support frame or base 110 is located at ground level in the center of the main outer housing 86. As shown in FIG. 19, the sub-frame generally indicated at 111 is suspended from the main frame 110 for relative vertical oscillatory motion by a conventional spring and bolt shock absorbing arrangement 119. An approximately one-sixteenth inch vertical oscillation, at preferably between about 500 to 800 cycles per minute, is generated by a conventional rotary eccentric mechanism indicated generally at 120 and driven by the motor 121. Eight spaced circularly arranged rigid metal straps 122 extend vertically upward from the annular base portion 123 of the sub-frame 111. The straps 122 are braced in spaced relation at their bottoms by gusset 9 plates 124 and above their mid-points by brace band 125.

The upper drying tray 106, a generally conically arranged series of eight upwardly converging plates, is supported on the eight rigid straps 122. The second drying tray 112, a generally conically arranged series of eight downwardly converging plates, is supported beneath upper tray 106 on the straps 122 by gusset plates (not shown). Second tray 112 is further supported by straps (not shown) depending from the radially outer portion of upper tray 106. A generally conical down wardly converging particle sorting screen 115 extends radially inwardly from the radially inner end of second tray 112 to the upper end of cylindrical conduit column 116. Conduit 116 is located inside the metal straps 122 and extends vertically upward from the annular base portion 123 of the sub-frame 111 to a position slightly below the mid-point of the straps 122. Conduit column 1 16 surrounds the lower portion of vertical duct 98 and its lower end is disposed to deposit material in the pulverizer 85. The third drying tray 113, a generally conically arranged series of eight upwardly converging plates, is supported beneath second tray 112 and the screen 115 from the straps 122.

The conical downwardly converging collector 114 Is supported by gussets 130 on the straps 122 beneath the third drying tray 113.

The radially outer edges of tray 112 and collector 114 extend beyond the vertical planes of the radially outer edges of the drying trays 106 and 113 so that material flowing downwardly off of trays 106 and 113 land on tray 112 and collector 114 respectively.

Movement of pulverized material down from the upper tray 106, across trays 112 and 113, screen 115 and collector 114, and into the hopper 126 is assisted by the vibratory motion of the sub-frame 111 transmitted to the trays 106, 112 and 113, screen 115 and collector 114. Trays 106, 112 and 113 are preferably disposed at angles approximately 30 from horizontal. The screen 115 is disposed at a sharper angle, preferably between 50 and 60 relative to horizontal.

Particles falling onto the screen 115 which are too large to pass through it will pass across the screen and be returned to the pulverizer 85 via conduit column 116. A deflector plate 127 is disposed inside conduit column 116 at its lower end. The lower end of deflector plate 127 terminates at opening 128 directly above return intake 129, so that particles entering conduit column 116 are directed through the opening 128 into the pulverizer 85 via return intake 129.

V Particles falling onto the screen 115 which are small enough to pass through it will drop onto third drying tray 113, then onto conical collector 114, and finally pass through discharge opening 128 into hopper 126.

The screen 115 is preferably constructed in sections which are removable and replaceable with similar screen sections having different sized mesh openings in order to alter maximum particle size in the end product.

Circular gas burners 117 are mounted in the drying chamber adjacent and surrounding the radially outer edge of the third drying tray 114. Heat from the burners ll7 passes upwardly across the drying trays 106,

"""112 and 114 and through the screen 115. As annular insulating shroud 118 depends from the radially outer edge of the second drying tray 112 so that heat from the burners 117 must pass through the screen 115 rather than escape around the outer edges of tray 112. In addition, the downwardly converging upper tray 106 extends out to a point near the side wall of the drier housing 86, thus enabling the upper tray 106 to trap or deflect a large portion of heat beneath it and reduce heat loss through the flue 131. A similar heat trapping effect beneath the third tray 113 is avoided, however, by providing exhaust openings 138 in the conduit column 116 adjacent its upper edge.

The flue 131 leads from the top of the drier housing 86, down through an afterburner stage 133, through a chamber 134 located beneath the mixing bin 81, and then up and out the chimney 135 located at the opposite end of the mixing bin 81 from the drier housing 86. Exhaust gases entering the flue 131 are forced downwardly into the afterburner 133 by a fan 136. Radiant burners 137 provide more complete combustion of end products from the drying process as the exhaust gases pass through the afterburner 133. When the exhaust gases leave the afterburner and enter chamber 134, they give off heat to material in the mixing bin, thereby pre-heating that material and effecting important economies in the efficient use of drying heat and fuel.

Further heat and fuel economies are provided by employing a heat exchanger on the chimney 135. One form of heat exchanger construction is shown generally at 139 and has a sleeve 140 coaxially surrounding and spaced from the chimney 135. The upper end of the sleeve 140 is open to the ambient atmosphere and the lower end communicates with a passage 141 leading back to the base of the drying chamber 143. Operation of the heat exchanger is actuated by a fan 142 located at the inlet to passage 141 on the chimmney 135 which draws air into the passage 141 and forces it through such passage, down along the sides of the chimney 135, absorbing heat therefrom, and back to the drying chamber 143.

Dried pulverizedmaterial entering the hopper 126 beneath the drying chamber 143 is transported by auger-type discharge conveyor 144 to the unloading conduit 146 which deposits finished, properly dried and pulverized material in a storage drum (not shown) or other container. The discharge conveyor 144 is also provided with a downwardly depending branch 147 upstream of the unloading conduit. This branch or return conduit 147 deposits material from the discharge conveyor 144 into the mixing bin 81. The lower end of the return conduit 147 terminates a predetermined distance below the horizontal plane of the upper edges of the mixing bin 81. As a result, when the level of material in the mixing bin 81 is above a predetermined level, the lower end of the return conduit 147 will be submerged and material in the discharge conveyor 144 will tend to fill the return conduit 147 to a level where it will pass across the return conduit, proceed to the unloading conduit 146, and be discharged from the system. When the level of material in the mixing bin 81 is below the lower end of the return conduit 147, material will tend to flow out of the discharge conveyor 144 into the mixing bin.

A temperature sensor 150, such as a thermocouple, is employed to sense the temperature of exhaust gases in the flue 131 at 151 adjacent the drying chamber 143, and such sensor is employed in conventional fashion to control the speed of the raw material conveyor responsive to temperature changes in the flue 131. The sensor 150 is biased to slow the raw material conveyor 80 when the temperature at 151 is below the temperature associated with properly dried material, and to increase the speed of raw material conveyor 80 when temperature at 151 is above that temperature level. In this manner, material in the discharge conveyor 144 will be returned to the mixing bin 81 when the temperature at 151 indicates that material in the system is not being dried to desired levels, and, conversely, material will be discharged from the system when the temperature at 151 indicates drying is proceeding properly.

An alternate to temperature sensor 150 is the amperage sensor 150a which senses the amperage drawn by hammermill drive motor 94. Raw material conveyor speed is made responsive to this amperage draw by conventional methods. When very wet material is in the hammermill 85, the hammermill motor tends to draw more current causing raw material conveyor speed to be reduced, in turn causing partially dried material in the discharge conveyor to be returned to the mixing bin via return conduit 147. Conversely, when relatively dry material is in the pulverizer 85, the pulverizer draws relatively little current, increasing raw material conveyor speed which'tends to slow return of partially dried material to the mixing bin.

A second alternate to temperature sensor 150 is the moisture sensor l50b which senses the moisture of material in the discharge conveyor 144 upstream of the return conduit 147. As in the two previous alternate embodiments, raw material conveyor speed is made responsive to sensor readings and conventional methods are employed to automatically slow the raw material conveyor when moisture content is sensed to be high.

Afterburner temperatures are preferably held within desired limits by employing a temperature sensor 152 which senses the temperature of the exhaust gases downstream from the burners 137 at 153 and, by conventional techniques, causes the operating temperature levels of the burners 137 to be adjusted automatically responsive thereto.

In the drying chamber 143, vibrator speed is manually set at the desired level with rheostat 154. Relatively greater vibration is required for relatively heavier materials.

Drier temperature is preferably controlled automatically with temperature sensor 155 which senses the temperature differential between the burner itself and the drying chamber at a point adjacent the lower radially inner edge of drying tray 112, as at 156. A thermocouple device is employed here and burner levels are controlled through valve 157 which is made responsive to the thermocouple by conventional methods.

In a typical operation, where the raw material consists essentially of chicken or cow manure, the temperature profile of the drier might appear as follows: 250-300F. on the upper tray 106; 700-800F. on second tray 112; 650-750F. at the screen 115; 275-325F. on the third tray 113; about 100F. above ambient temperature at the collector 114; and, about 50F. above ambient at the discharge end of the conveyor 144. In the exhaust flue, the temperature is about 200F. upstream of the afterburner 137, increasing to about 1,200F. at the afterburner, decreasing to about 800F. at the base of the afterburner section, and decreasing further to about 350F. at the stack outlet when the heat exchanger 139 is employed. Without the heat exchanger, the stack exhaust temperature typically ranges some 200F. higher (about 550F.).

The second tray 112 and the screen are in the peak temperature zone of the drying chamber 143. It will be noted that this is largely a result of the arrangement of the upwardly converging conical upper tray 106 which extends radially outwardly to a point quite near the side of the main housing 86. Since the heated air has a natural tendency to rise, much of it is trapped beneath the upper tray 106, thereby heating both the upper tray 106 and the zone beneath it wherein both the second tray 112 and screen 115 are located. Relatively small amounts of heat escape up past the radially outer edge of the upper tray 106, so that heat loss is minimized, thereby effecting important economies in fuel consumption.

Unlike some prior art devices, it will be noted that the drier of the present invention conducts all mixing and pulverization outside the drying chamber. In this manner, the time actually spent in the drier of the present invention is minimized and the nutrient content of the material is preserved. To the same end, the raw material is preheated at relatively low temperatures which do not materially affect nutrient content, but which further minimize the amount of time the material need be exposed to high temperature in the drier. Preheating occurs by admixture of dry, heated material with the raw material as occurs when return conduit 147 is actuated, and it occurs further by transfer of heat from the exhaust flue downstream of the afterburner when it passes beneath the mixing bin 81.

During operation of the device in drying typical animal waste products such as cow and chicken manure, I have discovered that a fluffy layer of indeterminate composition tends to build up on the interior of the roof of the housing 86. This fluff is beneficial in that it serves to insulate the drier roof and further reduce heat loss and fuel consumption.

Reference has been made to controlling minimum particle size. This may be done in several ways, perhaps the simplest of which is to branch the discharge outlet 146 so that all material leaving the machine must first tumble across an inclined screen. Particles failing to pass through such screen would be conducted via the first branch out of the system. Particles small enough to pass through the screen would be returned to the mixing bin 81 for reprocessing via the second branch. Actual construction of this and alternate features will be apparent.

Vapor generated during drying tends to collect in a cloud in the upper region of the drying chamber 143. I have found that it is important to draw off this vapor so that the cloud does not become so large as to extend down onto the drying trays where it would interfere with the drying process. Accordingly, the minimum speed of exhaust fan 136 should be great enough to prevent excessive vapor cloud build-up. Certain prior art driers, such as horizontal axis rotary drum driers, have typically exhausted both vapor and dried pulverized material through the same port, thereby exposing the dried material to moisture vapor. The drier of the present invention exhausts dried material from the bottom and vapor from the top so that there is minimal contact between vapor and material after drying has once occurred. Further in this regard, it is important to recognize that by minimizing flow-by heat loss in the drying chamber 143, as discussed earlier, turbulence in the drier is also minimized. As a result, water vapor is able to collect in a localized cloud at the top of the drying chamber away from the drying trays. With excessive turbulence, the vapor would be dispersed and could result in an increasedincidence of vapor contamination of the already dried material, thereby hindering the drying process.

It will be apparent to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof and, therefore, the invention is not limited by that which is shown in the drawings and described in the specification but only indicated in the appended claims.

I claim:

1. A drier for bulk material, said drier having a housing, heating means within said housing, a hopper within said housing, a pulverizer disposed within said housing and adapted to throw the discharge therefrom at substantial velocity, supply conveying means adapted to deliver material to said pulverizer, and discharge conveying means adapted to remove processed material from said hopper, wherein the improvement comprises:

a support frame disposed within said housing;

sub-frame mounted on said support frame for limited movement with respect thereto;

oscillating means operative to induce oscillating movement of said sub-frame with respect to said support frame;

a substantially vertical duct rotatably mounted in said housing and disposed to receive at the lower end thereof the discharge from said pulverizer, the upper extremity of said duct being laterally turned to discharge the material carried by said duct at a distance from the axis of rotation thereof;

a conduit column terminating at the upper end thereof below the upper extremity of said duct, and surrounding said duct and radially spaced therefrom, said column being disposed to deposit material in said pulverizer;

a downwardly converging conical receiver fixed with respect to said sub-frame and having a central opening-communicating with the interior of said column, said receiver including screen means radially outward from said column;

a conical downwardly converging collector fixed with respect to said sub-frame in vertically spaced relationship below said receiver, said collector having a central opening disposed to deposit material in said hopper;

annular shroud means interconnecting the peripheral portions of said receiver and collector;

baffle means fixed with respect to said sub-frame and adapted to vertically transfer material passing through said screen means to said collector; and

i return conduit means disposed to conduct material from said discharge conveying means to said supply conveying means.

2. A drier as defined in claim 1, wherein said subframe is resiliently mounted on said support frame, said column is mounted on said sub-frame, and said receiver, collector, and baffle means are secured to said column.

3. A drier as defined in claim 1, wherein said shroud means is in annular spaced relationship with said housing.

4. A drier as defined in claim 1, and additionally including a bin communicating with said supply conveyor and mixing means in said bin.

5. A'drier as defined in claim 4,additionally including a raw material conveyor adapted to discharge into said bin.

6. A drier as defined in claim 5, additionally including control means for said raw material conveyor responsive to a moisture-detecting device associated with said discharge conveyor.

7. A drier as defined in claim 4, wherein said return conduit is operative to transfer material exclusively in response to a level of material in said bin below a predetermined point.

8. A drier for bulk material, said drier having a housing, moisture-removal means within said housing and including pulverizing and screening means, a supplyconveyor adapted to deliver material to said housing, and a discharge conveyor adapted to remove processed material from said housing, wherein the improvement comprises;

return conduit means communicating between said discharge conveyor and said supply conveyor for conducting material from said discharge conveyor to said supply conveyor;

a raw material conveyor communicating with said supply conveyor and adapted to deliver material thereto;

means for driving said raw material conveyor; and

means for controlling said driving means responsive to the moisture content of material in said discharge conveyor, so that the rate at which raw material is conveyed by said raw material conveyor to said supply conveyor is reduced when the moisture content of material in said discharge conveyor increases.

9. A drier as defined in claim 8, additionally including a bin located at the intake end of said supply conveyor and at the discharge end of said raw material conveyor, and wherein said return conduit means has an upper end communicating with said discharge conveyor and a lower end communicating with said bin, said lower end terminating at a predetermined level above the floor of said bin, so that when the rate at which said material is conveyed by said raw material conveyor to said bin is approximately at least as great as the rate at which said material is conveyed away from said bin by said supply conveyor, the volume of material in said bin may be maintained above said predetermined level covering the lower end of said return conduit, but when the rate at which said material is conveyed by said raw material conveyor is reduced, the volume of material in said bin will fall below said predetermined level permitting material to flow from said return conduit into said bin.

10. In a drier for moisture-containing bulk material including a pulverizer adapted to receive and pulverize raw material, heating means for drying the discharge from said pulverizer, means for screening out from said dried material, oversize particles and returning these to said pulverizer and a hopper for receiving the dried particles that pass through said screening means, the improvement which comprises:

means rotatable about an upright axis for distributing the discharge from said pulverizer in a generally radial direction from said axis during rotation thereof, and

means defining sloping surfaces symmetrical about said axis for guiding said discharge downwardly across said surfaces in a generally radial inward and outward descending migration relative to said axis to be dried by said heating means.

11. Apparatus as defined in claim including means for imparting oscillating movement to said guide means to encourage said downward migration.

12. Apparatus as defined in claim 10 wherein said surfaces defined by said guiding means are frustoconical surfaces with the generatrix thereof centered at said axis.

13. Apparatus as defined in claim 10 including means for sensing the moisture level in the raw material being supplied to said pulverizer and means responsive to said moisture level for returning a portion of said dried particles from said hopper to said raw material to decrease the resultant moisture level in the raw material received by said pulverizer.

14. Apparatus as defined in claim 10 wherein said guiding means comprises a downwardly converging frusto-conical surface including said screen means, said surface having a central opening located above and communicating with said pulverizer whereby oversize particles pass through said opening and return to said pulverizer.

15. Apparatus as defined in claim 14 wherein said guiding means further includes a downwardly converging frusto-conical surface spaced below said screen means to receive particles passed by said screen means and having a central opening whereby particles are directed through said opening to said hopper.

16. Apparatus as defined in claim 10 wherein said distributing means comprises a substantially vertical duct disposed to receive at the lower end thereof, the discharge from said pulverizer, the upper extremity of said duct being laterally turned to discharge the material carried by said duct at a location radially spaced from the axis of rotation thereof.

17. Apparatus as defined in claim 10 wherein said distributing means comprises a distributor box spaced above and adapted to receive the material from said pulverizer, means mounted above said box for rotating said box, a deflector plate in said box angularly disposed relative to the axis of rotation of said box and adapted to deflect material thrown from said pulverizer, to the enclosed space above said deflector whereby said material migrates down the sloping top surface of said deflector to said guiding means.

18. Apparatus as defined in claim 10 wherein said guiding means comprises an upwardly converging frusto-conical surface adapted to receive material from said material from said distributing means radially outward and downward, and a downwardly converging frusto-conical surface spaced below said first-named suface and adapted to receive material therefrom, said second-named surface including said screen means and having a central opening located above and communicating with said pulverizer whereby oversize particles pass through said opening and return to said pulverizer.

19. Apparatus as defined in claim 18 wherein said guiding means further includes an upwardy converging frusto-conical surface spaced below said screen means and adapted to receive material passed through said screen means and a downwardly converging frustoconical surface spaced below said above-named surface to receive particles therefrom and having a central opening whereby particles are directed through said opening to said hopper.

20. Apparatus as defined in claim 10 including means for burning combustible gases contained in the gaseous product produced from said material by said heating means.

21. In a drier for moisture-containing bulk material including a pulverizer adapted to receive and pulverize raw material and to throw the discharge therefrom at substantial velocity, heating means for drying the discharge from said pulverizer and a hopper adapted to receive material dried by said heating means, the im provement which comprises:

means rotatable about a vertical axis for distributing the discharge from said pulverizer in a radial direction from said axis during rotation thereof, means defining sloping surfaces symmetrical about said axis for guiding said discharge downwardly across said surfaces in a generally radial inward and outward descending migration relative to said axis,

means for imparting oscillating movement to said guide means to encourage said downward migration,

screen means associated with said guiding means and adapted to screen out oversize particles and return them to said pulverizer and to pass therethrough dried particles of smaller size for collection in said hopper.

22. Apparatus as defined in claim 21 including means for adjusting the rate of feeding of raw material to said pulverizer in response to the temperature of dried ma- -terial in said hopper.

23. An improved method of drying a bulk material such as an organic waste material, comprising the steps of feeding the bulk material into a pulverizer, shredding the material within said pulverizer, screening the shredded material to separate the fine material from the coarse material, returning the coarse material to the pulverizer for shredding, heating the fine material to dry the material, sensing the condition of the dried fine material, returning at least a portion of the dried material to the bulk material fed into the said pulverizer, and controlling the proportion of dried material returned to the bulk material according to the condition of the dried material.

24. The method of claim 23 in which said sensing includes sensing of the moisture content of dried material.

25. ln a drier for bulk material, including a housing, means for supplying bulk material to said housing, means for pulverizing the material, means for screening the pulverized material, means within said housing for heating the pulverized screened material to remove moisture therefrom, means for discharging dried material from said drying means, and means for returning dried material from said material discharging means to said material supplying means to reduce the moisture content of the bulk material supplied to said housing, the improvement comprising sensing means for sensing the moisture content of the dried material, and means responsive to the sensing means for controlling the relative amount of dried material returned from said discharging means to said supplying means according to the moisture content of the dried materialdis cha'rged from said drying means. 

2. A drier as defined in claim 1, wherein said sub-frame is resiliently mounted on said support frame, said column is mounted on said sub-frame, and said receiver, collector, and baffle means are secured to said column.
 3. A drier as defined in claim 1, wherein said shroud means is in annular spaced relationship with said housing.
 4. A drier as defined in claim 1, and additionally including a bin communicating with said supply conveyor and mixing means in said bin.
 5. A drier as defined in claim 4, additionally including a raw material conveyor adapted to discharge into said bin.
 6. A drier as defined in claim 5, additionally including control means for said raw material conveyor responsive to a moisture-detecting device associated with said discharge conveyor.
 7. A drier as defined in claim 4, wherein said return conduit is operative to transfer material exclusively in response to a level of material in said bin below a predetermined point.
 8. A drier for bulk material, said drier having a housing, moisture-removal means within said housing and including pulverizing and screening means, a supply-conveyor adapted to deliver material to said housing, and a discharge conveyor adapted to remove processed material from said housing, wherein the improvement comprises; return conduit means communicating between said discharge conveyor and said supply conveyor for conducting material from said discharge conveyor to said supply conveyor; a raw material conveyor communicating with said supply conveyor and adapted to deliver material thereto; means for driving said raw material conveyor; and means for controlling said driving means responsive to the moisture content of material in said discharge conveyor, so that the rate at which raw material is conveyed by said raw material conveyor to said supply conveyor is reduced when the moisture content of material in said discharge conveyor increases.
 9. A drier as defined in claim 8, additionally including a bin located at the intake end of said supply conveyor and at the discharge end of said raw material conveyor, and wherein said return conduit means has an upper end communicating with said discharge conveyor and a lower end communicating with said bin, said lower end terminating at a predetermined level above the floor of said bin, so that when the rate at which said material is conveyed by said raw material conveyor to said bin is approximately at least as great as the rate at which said material is conveyed away from said bin by said supply conveyor, the volume of material in said bin may be maintained above said predetermined level covering the lower end of said return conduit, but when the rate at which said material is conveyed by said raw material conveyor is reduced, the volume of material in said bin will fall below said predetermined level permitting material to flow from said return conduit into said bin.
 10. In a drier for moisture-containing bulk material including a pulverizer adapted to receive and pulverize raw material, heating means for drying the discharge from said pulverizer, means for screening out from said dried material, oversize particles and returning these to said pulverizer and a hopper for receiving the dried particles that pass through said screening means, the improvement which comprises: means rotatable about an upright axis for distributing the discharge from said pulverizer in a generally radial direction from said axis during rotation thereof, and means defining sloping surfaces symmetrical about said axis for guiding said discharge downwardly across said surfaces in a generally radial inward and outward descending migration relative to said axis to be dried by said heating means.
 11. Apparatus as defined in claim 10 including means for imparting oscillating movement to said guide means to encourage said downward migration.
 12. Apparatus as defined in claim 10 wherein said surfaces defined by said guiding means are frusto-conical surfaces with the generatrix thereof centered at said axis.
 13. Apparatus as defined in claim 10 including means for sensing the moisture level in the raw material being supplied to said pulverizer and means responsive to said moisture level for returning a portion of said dried particles from said hopper to said raw material to decrease the resultant moisture level in the raw material received by said pulverizer.
 14. Apparatus as defined in claim 10 wherein said guiding means comprises a downwardly converging frusto-conical surface including said screen means, said surface having a central opening located above and communicating with said pulverizer whereby oversize particles pass through said opening and return to said pulverizer.
 15. Apparatus as defined in claim 14 wherein said guiding means further includes a downwardly converging frusto-conical surface spaced below said screen means to receive particles passed by said screen means and having a central opening whereby particles are directed through said opening to said hopper.
 16. Apparatus as defined in claim 10 wherein said distributing means comprises a substantially vertical duct disposed to receive at the lower end thereof, the discharge from said pulverizer, the upper extremity of said duct being laterally turned to discharge the material carried by said duct at a location radially spaced from the axis of rotation thereof.
 17. Apparatus as defined in claim 10 wherein said distributing means comprises a distributor box spaced above and adapted to receive the material from said pulverizer, means mounted above said box for rotating said box, a deflector plate in said box angularly disposed relative to the axis of rotation of said box and adapted to deflect material thrown from said pulverizer, to the enclosed space above said deflector whereby said material migrates down the sloping top surface of said deflector to said guiding means.
 18. Apparatus as defined in claim 10 wherein said guiding means comprises an upwardly converging frusto-conical surface adapted to receive material from said material from said distributing means radially outward and downward, and a downwardly converging frusto-conical surface spaced below said first-named surface and adapted to receive material therefrom, said Second-named surface including said screen means and having a central opening located above and communicating with said pulverizer whereby oversize particles pass through said opening and return to said pulverizer.
 19. Apparatus as defined in claim 18 wherein said guiding means further includes an upwardly converging frusto-conical surface spaced below said screen means and adapted to receive material passed through said screen means and a downwardly converging frusto-conical surface spaced below said above-named surface to receive particles therefrom and having a central opening whereby particles are directed through said opening to said hopper.
 20. Apparatus as defined in claim 10 including means for burning combustible gases contained in the gaseous product produced from said material by said heating means.
 21. In a drier for moisture-containing bulk material including a pulverizer adapted to receive and pulverize raw material and to throw the discharge therefrom at substantial velocity, heating means for drying the discharge from said pulverizer and a hopper adapted to receive material dried by said heating means, the improvement which comprises: means rotatable about a vertical axis for distributing the discharge from said pulverizer in a radial direction from said axis during rotation thereof, means defining sloping surfaces symmetrical about said axis for guiding said discharge downwardly across said surfaces in a generally radial inward and outward descending migration relative to said axis, means for imparting oscillating movement to said guide means to encourage said downward migration, screen means associated with said guiding means and adapted to screen out oversize particles and return them to said pulverizer and to pass therethrough dried particles of smaller size for collection in said hopper.
 22. Apparatus as defined in claim 21 including means for adjusting the rate of feeding of raw material to said pulverizer in response to the temperature of dried material in said hopper.
 23. An improved method of drying a bulk material such as an organic waste material, comprising the steps of feeding the bulk material into a pulverizer, shredding the material within said pulverizer, screening the shredded material to separate the fine material from the coarse material, returning the coarse material to the pulverizer for shredding, heating the fine material to dry the material, sensing the condition of the dried fine material, returning at least a portion of the dried material to the bulk material fed into the said pulverizer, and controlling the proportion of dried material returned to the bulk material according to the condition of the dried material.
 24. The method of claim 23 in which said sensing includes sensing of the moisture content of dried material.
 25. In a drier for bulk material, including a housing, means for supplying bulk material to said housing, means for pulverizing the material, means for screening the pulverized material, means within said housing for heating the pulverized screened material to remove moisture therefrom, means for discharging dried material from said drying means, and means for returning dried material from said material discharging means to said material supplying means to reduce the moisture content of the bulk material supplied to said housing, the improvement comprising sensing means for sensing the moisture content of the dried material, and means responsive to the sensing means for controlling the relative amount of dried material returned from said discharging means to said supplying means according to the moisture content of the dried material discharged from said drying means. 